The present invention used in a multi-color printing machine refers to a device for scanning various color marks printed by various printing units, the marks allowing a subsequent determination of misregister of a corresponding color with respect to the color printed by the first printing unit and used as a reference.
Known devices such as the one described in the document EP 0 094 027, corresponding to U.S. Pat. No. 4,534,288, operate to satisfaction, provided that the yellow, blue or red marks are sufficiently contrasted for being recognized infallibly by the scanning device. Some of these known devices may operate with a fiber optic light beam hitting the printed workpiece and directing the reflected light onto a scanning photodiode producing an electric signal. In order to enhance the contrast between the electric basic signal corresponding to a non-printed area of the workpiece, and an electric impulse produced by the passing mark, a filter, usually of blue color, is installed between the fiber optic and the photodiode.
However, as soon as the printed colors fade to paleness, especially so when printing packages with pastel yellow, cream or light blue, the conventional devices are no longer capable of detecting safely the various printed marks so that the one or the other register control might fail to operate correctly. In such cases, it might be appropriate to use a first filter, to present a pale color in order to test the quality of the signal obtained, and to repeat the test with one or several other filters so as to select the one most appropriate for all marks. However, the most important phase of the start-up of a printing machine consists in searching the initially unknown position of a concomitant mark which, though, cannot be reliably carried out without an immediately responsive scanning device. Such numerous, and indispensible, tests become quickly inhibitive if the printing machine is to be used for accomplishing a great number of different jobs.